1. Field of the Invention
The present invention relates to a method for producing a polymer organic electronic material; a polymer organic electronic material which can be obtained by the method; and an organic electroluminescent device which uses the polymer organic electronic material.
2. Description of the Related Art
Organic electroluminescent devices (hereinafter, referred to as “organic EL devices”) are promising for a wide range of applications because they are self-luminescent fully solid-state elements with a high visibility, and are also strong against impact. Currently, although those which use inorganic fluorescent materials are predominant, there are problems in that the production cost is high due to a necessity for alternating voltage of 200 V or more for operation, and the brightness is insufficient.
On the other hand, organic EL device research using organic compounds initially began by using single crystals such as anthracene and the like, although in the case of a single crystal, the film thickness was thick at approximately 1 mm, and a driving voltage of 100 V or more was required. For this reason, thinning of the films by vapor deposition methods is being attempted (refer to, for example, Thin Solid Films, Vol. 94, 171, (1982)).
However, the thin-films obtained by this method still have a high driving voltage of 30 V. Furthermore, since the density of electron and hole carriers within the film is low, and the generation probability of photons from the recombination of carriers is low, adequate brightness could not be obtained.
However, in recent years, function separated-type organic EL devices in which thin-films of a hole transporting organic low molecular weight compound and a fluorescent organic low molecular weight compound that possesses an electron transporting functionality are sequentially laminated by a vacuum deposition method, wherein a high brightness of 1000 cd/m2 or more can be obtained at a low voltage of approximately 10 V, have been reported (refer to, for example, Appl. Phys. Lett., Vol. 51, 913 (1987)). Since then, research and development of laminated-type organic EL devices have been actively carried out.
However, in this type of organic EL device, since a thin-film of 0.1 μm or less is formed in plural deposition processes, pinholes tend to occur. In order to obtain sufficient performance, it is necessary to control film thickness under strictly managed conditions. Therefore, there are problems in that productivity is low, and area enlargement is difficult. Furthermore, since this organic EL device is operated at a high current density of a few mA/cm2, Joule heat is generated in large quantities. Consequently, films of the hole transporting organic low molecular weight compound, the fluorescent organic low molecular weight compound, and the like, which are produced in an amorphous glass state by deposition, gradually become crystalline and ultimately melt, and phenomena, wherein the brightness decreases and dielectric breakdown occurs, are commonly observed. As a result, there is a problem in that the lifetime of the element is reduced.
For example, in the case of the electron transporting material, although the use of oxadiazole derivatives, including 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBO), as an electron transporting material has been proposed (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-109454), the obtained thin-film has a tendency to crystallize, and also from the aspect of charge transport and injection characteristics, it cannot be said that these are sufficient. Moreover, there are very few types of electron transporting materials other than oxadiazole compounds, and further development of superior materials from the aspect of low voltage operation and high efficiency, as well as charge transport and injection characteristics, is desired.
On the other hand, research and development of single layer structured organic EL devices have been carried out with an aim to solve the problems relating to productivity and area enlargement in laminated-type organic EL devices. Moreover, devices using conductive polymers such as poly(p-phenylene vinylene) (refer to, for example, Nature, Vol. 357, 477 (1992)), and devices in which an electron transporting material and a fluorescent pigment blended within hole transporting polyvinyl carbazole (refer to, for example, Proceedings of the 38th Meeting of the Japan Society of Applied Physics and Related Societies 31p-G-12, 1991) have been proposed. However, brightness, light emitting efficiency, and the like, have not yet reached levels of the laminated-type organic EL devices using organic low molecular weight compounds.
The materials used in organic EL devices are required to be high purity materials in order to ensure long-term stability of the electrical characteristics and device lifetime. Accordingly, the materials used in organic EL devices are required to be highly purified by a purification process following synthesis. In particular, in the case of a polymer material, this requirement is attempted to be met by subjecting the polymer obtained following polymerization to solvent washing, or washing processes of various types by acid cleaning, alkaline cleaning, and the like, or by repetition of a reprecipitation process by dissolution in a good solvent followed by dropwise addition into a poor solvent. However, purification of polymer materials is generally difficult compared to low molecular weight compounds, and improvement in the electrical characteristics of organic EL devices following these processes was insufficient. Furthermore, the presence of the smallest amounts of impurities in the charge transporting polymer material used in organic EL devices greatly influenced the device, resulting in deterioration in electrical characteristics and lifetime of the device.
A process wherein dissolution in a good solvent and reprecipitation in a poor solvent is repeated has been disclosed (refer to, for example, Japanese National Publication No. 8-510285), although it was not possible to completely remove the unreacted monomers and the decomposed impurities produced at the time of polymerization, even with a high repetition frequency.
Furthermore, a purification method wherein Soxhlet extraction by methanol is performed has been disclosed (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-216965). However, in an extraction where a poor solvent such as methanol is used, due to low solubility, it was not possible to completely remove the unreacted monomers and the decomposed impurities.
A purification method wherein the low molecular weight components are removed by dialysis has been disclosed (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-138133), although it is only applicable to water-soluble polymers, and purification effects could not be observed for hydrophobic polymers.
Inventions which produce polymer hole transporting materials or light emitting materials by a filtration method have been disclosed (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-41982 and JP-A No. 2005-44615). This filtration method is one in which the hole transporting material or the light emitting material is prepared by passing the solution through a filter. However, the method was unsuitable for processing a large number of samples at once.